1. Field of the Invention
The present invention relates to a substrate that constitutes an ink jet head (hereinafter, simply referred to as an ink jet head) for discharging functional liquid, such as ink, to recording media including paper sheet, plastic sheet, cloth, commodity, and the like, in order to record and print characters, symbols, images, and the like, while executing related operations. The invention also relates to a method for manufacturing such substrate, and an ink jet head formed by use of such substrate, as well as to a method for manufacturing such head.
2. Related Background Art
The ink jet recording method has, in recent years, attracted more attention because it operates more suitably for recording images in higher precision at higher speeds, while with this method, the recording head and apparatuses are made smaller and suitably adaptable for recording in color. (For example, refer to the specifications of U.S. Pat. Nos. 4,723,129 and 4,740,796.)
FIG. 1 is a view which shows the general structure of the principle part of the head substrate used for an ink jet recording head described above in accordance with one embodiment of the present invention.
In FIG. 1, the ink jet recording head is provided with a plurality of discharge openings 1001. Also, on the substrate 1004, the electrothermal transducing devices 1002 that generate thermal energy to be utilized for discharging ink from these openings are arranged for each of the ink flow paths 1003, respectively. Each of the electrothermal transducing devices are formed mainly by the heat generating member 1005, the electrode wiring 1006 that supplies electric power to it, and an insulation film that protects them.
Also, each of the ink flow paths 1003 are formed by a ceiling plate having a plurality of flow path walls 1008, which is adhesively bonded, while its relative positions to the electrothermal transducing devices and others on the substrate 1004 are adjusted by means of image processing or the like. The end of each of the ink flow paths 1003 on the side opposite to the discharge opening 1001 is conductively connected with a common liquid chamber 1009. In this common liquid chamber 1009, where ink supplied from an ink tank (not shown) is retained. Ink supplied to the common liquid chamber 1009 is conducted to each of the ink flow paths 1003 from the chamber, and is held in the vicinity of each discharge opening by means of a meniscus that the ink forms in such portion. At this juncture, when the electrothermal transducing devices are selectively driven, ink on the heat activation surface is abruptly heated by the utilization of thermal energy thus generated to bring about film boiling. Ink is discharged by means of its impulsive force at that time.
FIG. 2 is a cross-sectional view of the substrate for use of an ink jet recording head, taken along line 2--2 corresponding to an ink path represented in FIG. 1.
In FIG. 2, a reference numeral 2001 designates a silicon substrate, and 2002, a heat accumulation layer. A reference numeral 2003 designates an interlayer film formed by SiO film, SiN film, or the like, which dually functions to accumulate heat; 2004, a heat generating resistive layer; 2005, a metal wiring formed by Al, Al--Si, Al--Cu, or the like; and 2006, a protection layer formed by SiO film, SiN film, or the like. Also, a reference numeral 2007 designates an anti-cavitation film that protects the protection film 2006 from the chemical and physical shocks following the heat generation of the heat generating resistive layer 2004. Also, a reference numeral 2008 designates the heat activating portion of the heat generating resistive layer 2004.
Now, this heat activating portion is formed by the heat generating resistive layer 2004, the protection layer 2006 that protects the heat generating resistive layer 2004 from ink, and the interlayer film 2003 that gives thermal energy generated by the heat generating resistive layer to ink efficiently.
The heat activating portion of the ink jet head is under a severe environment, such as receiving mechanical shocks resulting from the cavitation caused by the repeated foaming and defoaming of ink; being exposed to erosion; and also, exposed to the considerable degree of temperature changes, up and down, in an extremely short period of 0.1 to 10 .mu.sec, among some other severe conditions. Therefore, the stabilization characteristics of the heat generating resistive layer 2004 itself, the characteristics of the protection layer 2006 and the interlayer film 2003 that sandwich the heat generating resistive layer 2004, with respect to the environment under which these elements are used, are the important factors that determine the performance of the ink jet head, such as its discharge stability and life.
As the heat generation resistive layer 2004 used for the ink jet head described above, TaN film, HfB.sub.2 film, or the like is generally used at present. Here, it is known that the stabilization characteristic of the heat generation resistive layer, particularly the rate of resistance changes at the time of repeated recording for a long time, depends largely on the composition of the TaN film. Of the heat generating members, it is known that the one formed by tantalum nitride which contains TaN.sub.0.8hex has a smaller rate of resistance changes at the time of repeated recording for a long time as described above, and that it is excellent in its discharge stability (see Japanese Pat. Laid-Open Application No. 7-125218).
Also, for the protection layer and the interlayer film used for the ink jet head described above, it is required to provide excellent capability of heat resistance, stable oxidation, insulation, resistance to breakage, and close contactness with the heat generation resistive layer. At present, SiO.sub.2, SiN, or some other inorganic compound is used in general.
In recent years, ink jet printers have been developed rapidly and put on the market widely. Along with such development, it is required to provide recorded images of higher precision. In order to meet such demand for higher precision of recorded images, there may be cited a method of making the size of ink droplets smaller still. To this end, the heat generating members should be provided with higher resistance. However, the limit of the specific resistance value of the material used for the conventional heat generating members described above is approximately 200 to 300 .mu..OMEGA..multidot.cm. No sufficient resistance value is obtainable or this particular purpose. Then, if many heat generating members conventionally in use should be arranged to meet the requirement of highly precise recording, the electric current value becomes very high because of the inability of obtaining sufficient resistance value. A great load is given to the heat generating members, making its life much shorter.
Also, it is required to record at higher speeds, because such highly precise image recording may considerably increase the numbers of droplets to be discharged. Consequently, the heat generating members should be driven at a high temperature in a shorter period of time at high speeds. This requires each of the heat generating resistive layers to provide more stabilized discharge capability, and thermal stability as well.
For the ink jet recording head, short pulses should be given at a high temperature to heat ink to be foamed and discharged. Therefore, the protection layer 2006 and the interlayer film 2003 are heated to considerably high temperatures due to the heat which is generated by the heat generating member 2004. Further, there are some cases that the interfaces of the protection layer and interlayer film or the portions having weaker film texture are damaged locally due to heat generated by the heat generating member following the repeated cycle of heating and cooling. Then, if electric power should be applied in shorter pulses in order to attempt the high-speed operation of the ink jet recording head, there are some cases that ink enters such interfaces or portions resulting in electric erosion, and leading to the problem that breakage of the heat generating resistive layer 2004 takes place.
Meanwhile, it is proposed in the specification of Japanese Patent Laid-Open Application No. 5-338175 that at least the portions of the heat generating resistive layer 2004 on the interfaces with the protection layer 2006 and the interlayer film 2003 are made to contain the materials of the protection layer 2006 and the interlayer film 2003 as the components of the material that forms the heat generating resistive layer, and that the components of material of the heat generating resistive layer are made to vary in the film thickness direction. In this manner, the thermal stress, which is caused by the difference in the thermal expansion coefficient, may be reduced at each interface between the layers, thus attempting the enhancement of its durability against the thermal stress.
However, in accordance with the structure proposed as described above, the formation thereof is made by the material of the heat generating member having crystal structures. As a result, the central portion of the heat generating member is formed only by such material in the film thickness direction, and the film thickness is made thinner because the specific resistance value is also low. Inevitably, therefore, this structure necessitates more rigid control in making the required film formation. At the same time, the temperature gradient of the heat generating member becomes greater in the film thickness direction. As a result, the proposed formation of the structure cannot meet the requirements sufficiently when smaller-sized heat generating members should be driven at higher speeds as described above.